1. Technical Field of the Invention
The present invention relates generally to an on-vehicle airbag system for protecting occupants by inflating an airbag in a car upon collision.
2. Background Art
Cars are essential to modem life, and an airbag has also become essential for protecting an occupant. A circuitry of a related airbag system (Japanese Patent First Publication No. 9-240416) is shown in FIG. 1. As shown, the airbag system 100 comprises a Cpu (Central Processing Unit) 101, am IC (Integral Circuit) 102 having a first switching element 106 and a second switching element 107, a mechanical sating sensor 103, an acceleration sensor 104 (hereinafter, simply referred to as “G-sensor”), a safing switching element 105, and a squib 120.
A series of operation procedures upon collision is described below. A signal from the mechanical safing sensor 103 is sent to the CPU 101 through an I/O circuit 108. If the CPU decides to choose ON, an approval signal is issued, and sent to an AND gate 110 through a signal line L100. The approval signal is also sent to an AND gate 111 through a signal line L101.
On the other hand, a detected signal from the G-sensor 104 is sent to the CPU 101 through an A/D (Analog to Digital) converter 109. If the CPU 101 chooses to squib, a driving signal is issued and sent to the AND gate 111 through a signal line L102. If both of the driving signal and the approval signal are sent to the AND gate 111, an AND signal is sent to a SCI (Serial Communication Interface) 112 from the AND gate 111. The AND signal is sent to the SPI (Serial Parallel Interface) 113 through a serial transmission channel 119, and the AND signal is sent to the AND gate 110 through a signal line L104. If both of the AND signal and the approval signal are sent to the AND gate 110, a safing switching element 105 is activated.
The driving signal is also sent to a SCI 114 through a signal line L103. The driving signal is sent to a SPI 116 in the IC 102 through a serial transmission channel 115. The driving signal from the SPI 116 is sent to a driving circuit 117 through a signal line L105. The first switching element 106 is driven by the driving circuit 117. The driving signal from the SPI 116 is also sent to a driving circuit 118 through a signal line L106. The second switching element 107 is driven by the driving circuit 118. If the safing switching element 105, the first switching element 106, and the second switching element 107 are all driven (that is, all of the elements are made ON), an electric current flows in an electric power line L107. A squib 120 generates heat on account of the electric current, and an inflator is fired up, resulting in inflation of an airbag in a car.
However, the airbag system 100 has only one channel 115 to connect the CPU 101 and the switching elements 106 and 107 in the IC 102. (The channel 119 connects only the CPU 101 and the AND gate 110.)
Because there is only one channel 115, if the channel 115 is effected by, for example, an external noise, it is likely to cause a problem on both of the first switching element 106 and the second switching element 107 simultaneously. Thus, an operation reliability of the airbag system is low.